The present invention relates to methods and apparatus for quantitatively measuring or characterizing the reception quality of a received frequency-modulated ultrashort-wave signal.
Because broadcast frequency-modulated ultrashort-wave signals spread out in space as they are propagated, such a signal when received by a particular receiving antenna will comprise not only a directly transmitted component but also, superimposed thereon, signal components transmitted to the receiving antenna indirectly as a result of reflection of the broadcast waves off of reflecting structures, such as buildings, geological structures and the like. Those signal components attributable to such reflection of the broadcast waves can be considered as constituting a noise signal which modulates the received IF carrier with respect to both amplitude and phase, in a sense of course detracting from reception quality.
Quantitative measurement or characterization of the quality of reception in such circumstances is extremely important in many instances, for example where that information is to be relied upon for steering or adjusting the receiving antenna. Methods and systems serving this purpose are known; attention is directed, by way of example, to the British Broadcasting Corporation research report BBC RD 1975/33.
With such conventional techniques, the amplitude modulation of the frequency-modulated carrier resulting from broadcast-wave reflections is ascertained as a function of frequency or frequency deviation of the carrier, and the quantitative reception quality characterization is derived from that relationship, in a manner discussed more fully below.
As already stated wave-reflection phenomena can be considered to act as a noise signal serving to modulate the frequency-modulated carrier with respect to both amplitude and phase. In general, it is not possible, i.e., during the actual reception of an ordinary ultrashort-wave FM broadcast, to continually ascertain with any meaningful precision the degree to which the carrier has become phase-modulated by the wave-reflection noise signal, because it is not possible to separately ascertain what phase variations, or components thereof, of the carrier signal are to be associated with the actual message signal, on the one hand, and the wave-reflection noise signal, on the other hand.
In contrast, it is possible to quantitatively ascertain the degree to which the frequency-modulated carrier has become amplitude-modulated by wave-reflection noise, for the simple reason that ultrashort-wave FM broadcasters of course take pains to assure that their transmitted carriers are of constant amplitude. Accordingly, at the receiver end, any amplitude fluctuations exhibited by the carrier can only be the result of noise.
In the already mentioned British Broadcasting Corporation research report BBC RD 1975/33 ("A field strength measuring receiver for band II"), a signal whose instantaneous value indicates the instantaneous amplitude of the frequency-modulated carrier is applied to the vertical-deflection input of an oscilloscope screen, and a signal whose instantaneous value indicates the instantaneous frequency or instantaneous frequency deviation is applied to the horizontal-deflection input. The resulting waveform generated on the oscilloscope screen displays the functional dependence of the frequency-modulated carrier's amplitude upon the frequency of the frequency-modulated carrier. The oscilloscope screen itself is provided with a horizontal axis directly calibrated in units of frequency. The oscilloscope screen is furthermore provided with a vertical axis directly calibrated in units of percentage amplitude modulation relative to the amplitude of the IF carrier if no wave-reflection noise phenomena were involved. E.g., when the ordinate value of the displayed graph is coincident with the horizontal axis of the display screen, this means that, here, the amplitude of the carrier equals 100% of the amplitude which the carrier would have if all wave-reflection modulating noise signals could somehow be removed. Where the ordinate value of the displayed curve is located above the horizontal axis, this means that the wave-reflection noise signals are decreasing the amplitude of the IF carrier relative to the imaginary reflection-free situation.
It is to be noted that merely in generating this displayed curve a very troublesome difficulty arises. In particular, the analyst must so adjust the gain of the y-deflection-input or amplitude-indicating signal that the horizontal reference axis implied by it coincide with the horizontal reference axis provided on the display screen. I.e., the horizontal reference axis on the display screen, considered as the 100% line for ordinate values of the curve, is to represent the ordinate value of the curve at which the actual amplitude of the received carrier coincides with the amplitude value which would exist in the imaginary reflection-free case. The horizontal reference axis implied by the displayed curve is the horizontal line joining all points on the curve whose ordinate values actually coincide with the amplitude which the carrier would have in the imaginary reflection-free case. In actual practice, adjusting the gain of the y-deflection-input or amplitude-indicating signal so that the implied horizontal reference axis coincides with the screen's horizontal reference axis proves to be extremely laborious and time-consuming.
The known system makes an attempt to deal with this problem. In particular, the known system uses negative-feedback control of the gain of the y-deflection or amplitude-indicating signal, to maintain constant the average (with respect to time) value of such signal. The analyst allows this automatic average-value negative-feedback regulation to operate for a while and then, at a suitable moment, presses a button which causes the average-value regulation to cease and the most recently established gain value for the amplitude-indicating signal to be locked in or held during the next part of the analyst's work. The working premise underlying this known technique is that by stabilizing (i.e., keeping constant) the average (with respect to time) value of the amplitude-indicating signal, there will be achieved a balancing out or cancellation of the opposite effects which the reflected-wave noise has on the amplitude of the carrier. I.e., whether the reflected-wave noise amplitude-modulates the carrier in a sense increasing its amplitude, or in a sense decreasing its amplitude, depends of course upon the phase of the reflected waves received by the antenna, and the instantaneous phase of the reflected waves varies with the instantaneous value of the actual message signal of interest. Of course the instantaneous value of the message signal will in general, during an ordinary ultrashort-wave FM broadcast, vary, i.e., depending upon message content, whether music or speech is involved, and so forth. Accordingly, merely to rely on the average (with respect to time) value of the carrier's amplitude is to rely upon a very inconstant quantity, and is merely to hope that the averaging-out process will itself balance out or cancel the opposite effects exerted from one moment to the next (or indeed from one broadcast program to the next) upon the amplitude of the carrier.
The very simple drawback of this known technique is that in fact, as a generality, the horizontal reference axis implied by the displayed amplitude-versus-frequency curve does not actually coincide with the horizontal reference axis provided on the oscilloscope screen. It will be understood that this lack of registration is a serious problem which goes beyond inconvenience in reading-off values from the screen, i.e., goes beyond the need for "mental" subtraction or addition to compensate for this lack of registration. Instead, when the 100%-ordinate-value horizontal reference axis implied by the displayed curve fails to coincide with that on the screen, the analyst must attempt to perform a computed conversion of actual scale, before he can quantitatively interpret the displayed curve. Furthermore, before the analyst can even attempt to compute such a scale conversion, he must know where the horizontal reference axis implied by the displayed curve is located, and such location is not always self-evident. Besides all this, there is the additional problem that, after the analyst has computed the necessary scale conversion and performed his quantitative reception-quality characterization, he cannot then proceed to vary the basic reception situation with which he is dealing without then losing what he has just achieved. For example, if the quantitative reception-quality characterization derived by the analyst indicates poor reception quality, and if as a result the analyst commands that the receiving antenna be steered to a different direction, this will, in general, basically alter the effects of the wave-reflection noise phenomena upon the carrier, and the whole procedure must in general be repeated from the start. I.e., after performing the quantitative analysis with the antenna aimed in a first direction, the analyst cannot simply rotate the antenna and immediately know whether this has improved or decreased the reception quality.